I. Field of the Invention
The present invention relates generally to mixing drums for batch-style tilt mixers generally and, in particular, those used to blend dry ingredients with water such as in mixing batches of concrete for loading transit carriers including concrete mixer trucks, commonly known as ready-mix trucks. The present invention relates directly to improvements in the drum which reduce the escape of unwanted dust associated with the loading and agitation of the dry ingredients during the using period in which the cement, aggregate and water combine to form concrete.
II. Discussion of the Related Art
Tilt mixers having drums of various types have been used for many years in the construction industry for mixing batches of aggregate, including stone and sand, with finely divided Portland cement and water to form concrete prior to loading the mixture into trucks for transport to the location to where it is to be placed. Over the years, experience has shown it to be most economical to manufacture tilt mixers in a size capable of fully loading a large ready-mix truck but also capable of being themselves transported by truck utilizing the highway system and thereafter mounted for use at an installed mixing plant. Tilt mixers are typically installed in elevated positions so that trucks hauling premixed materials may be loaded from above by tilting the mixer and discharging the mixed contents into a chute above the charging end of the concrete delivery truck.
Tilt mixers are normally designed to be charged to conduct a mixing operation with the longitudinal axis in a substantially horizontal position. In normal batching plant installations, the charging end of the drum closely addresses a fixed loading chute through which it receives measured amounts of aggregate, including stone and sand, Portland cement, and water according to any desired predetermined recipe. The charging end of the drum must be configured so as to clear the chute when the drum is tilted to discharge the mixed materials from the front opening into the discharge chute. The clearance with respect to the front or discharge end of the mixing drum is also important with respect to the loading of the materials into the trucks. Consideration of the height necessary to mount the drum for tilt loading of ready-mix trucks and the clearance for the charging chute are important considerations which must be weighed together with the desire to make the drum itself as close as is reasonable to the ideal mixing shape, in which the length of the drum equals or approaches the diameter of the drum.
Tilt mixers of the class described are usually operated by charging the dry ingredients into the drum through the charging chute and measured amount of water for the batch as the mixing drum is rotated. The dry materials include quantities of stone, sand and finely divided cement which tend to be dusty; and, because the dry ingredients may be agitated for some time before being thoroughly wet by the water in the mix, a great deal of airborne dust may be produced by the loading of the dry ingredients and during the initial stages of mixing. The charging and discharging accesses of the drum are normally rather large round uncovered openings and, for this reason, a great deal of dust may be generated and escape especially from the front or discharge end as the air in the hollow mixing chamber becomes dust laden and is forced out of the discharge opening as entering ingredients displace it.
In this regard various attempts have been made in the prior art to provide the front or discharge end of tilt mixers with dust hoods. These, for the most part, have been devices which are moved into place at the front of the mixing drum for use during mixing but which have to be removed prior to discharging the drum because the space occupied by the dust hood is needed for tilting the drum for discharge of the mixed batch. The need to remove the dust hood prior to discharging the batch also means that there is no hood or other device which might be present to prevent parts of the structure or other nearby devices from being occasionally spattered by wet mixed concrete which may splash out of the discharge chute.
With environmental considerations becoming more and more important in all types of industries, reduction of unwanted airborne dust, which can result in unwanted coatings of cement particles in everything throughout the mixing plant area, and also cause unwanted airborne particles in the local ambient air, has become a necessity. There has long been a need to provide an integral dust hood for concrete mixing devices which would both minimize the amount of dust produced from a discharge end of the tilt mixer drum produced by loading and in the initial stages of concrete mixing. It is also desirable that it be one which would prevent damage or other undesirable consequences of the splashing of mixed concrete during discharge of the drum.